Physical activity is an integral component of healthy aging among older adults. The social and built characteristics of the neighborhood environment influence this population's ability to walk and engage in moderate exercise. In spite of this knowledge, interventions designed to enhance physical activity in this population have not assessed the environmental factors that influence changes in physical activity behavior. Understanding modifiable environmental factors that affect the mechanisms of change in physical activity behavior is necessary for developing effective interventions that increase walking and moderate physical activity among older adults. The two goals of this research are: (1) to quantify the moderating effect of neighborhood accessibility on change in walking behavior; and (2) to evaluate the mediating effect of social cohesion on change in walking activity. Neighborhood accessibility, a component of the built environment describing the ease of reaching needed or desired activities, will be measured using geographical information systems (GIS). Social cohesion, a neighborhood-level psychosocial resource, is defined by high levels of social trust and shared values. To achieve the research goals, existing data from a neighborhood-based walking intervention for older adults will be linked to measures of neighborhood accessibility. Hierarchical linear models will be used to re-analyze this data. The results of this research are an important first step in understanding the factors that lead to a successful physical activity intervention program among the elderly. The use of existing data from a unique intervention study makes this study feasible and economical. Neighborhood-based interventions that target social cohesion will result in greater levels of physical activity among seniors and, as a consequence, will contribute significantly to increased physical health including the prevention or delay of the onset of disability in the elderly.